DESCRIPTION (Applicant's Abstract): The enzymes, receptors, and inhibitors comprising the plasminogen activator (PA)-plasmin fibrinolytic system contribute importantly to the pathogenesis of inflammation and tissue repair. Evidence is accumulating to indicate that urokinase-type PA (uPA) and uPA receptors (uPAR; CD87) are integral to leukocyte activation, and complex mechanisms have been described whereby uPAR participates in regulating cellular adhesion, migration, and cytokine activation. There is equally compelling evidence that binding uPA to uPAR directly activates signaling pathways, and our recent work has shown that uPAR aggregation, seen during leukocyte adhesion and migration, also initiates activation signaling. Still, our understanding of the mechanisms by which uPAR regulates leukocyte function is fragmentary. The central hypothesis underlying the proposed work is that uPA and uPAR serve pivotal functions in regulating the early events that transition neutrophils and mononuclear phagocytes from quiescent to "activated" phenotypes. The long-term objective of this project is to understand the roles uPA and uPAR play in leukocyte activation signaling, and to characterize the underlying pathways of signal transduction. The short-term objectives are to: i) determine the regulatory roles of uPAR aggregation on leukocyte activation signaling, emphasizing cooperative signaling with adhesion molecules, degranulation, superoxide release, and uPAR turnover; ii) determine whether uPA binding vs. aggregation causes formation of distinctive multiprotein signaling assemblies that partition into specialized lipid-rich domains of the plasma membrane; and iii) determine the role of uPAR in "outside-in" signaling by adhesion molecules, emphasizing signaling intermediates required for adhesion and migration. Thus far, our studies have fortified our hypothesis that uPAR-mediated signaling profoundly affects the function of neutrophils and mononuclear phagocytes. Hopefully, a more complete appreciation of the mechanisms and consequences of uPAR-mediated signaling will create new opportunities for therapeutically manipulating the function of these leukocytes in acute and chronic inflammation, wound healing, and fibrosis.